Tomographic image generating apparatus, tomographic image generating method, and tomographic image generating program

ABSTRACT

An image acquisition unit acquires a plurality of projection images corresponding to a plurality of radiation source positions in a case of tomosynthesis imaging. A reconstruction unit reconstructs all or a part of the plurality of projection images to generate a tomographic image on each of a plurality of tomographic planes of a subject. A feature point detecting unit detects at least one feature point from a plurality of the tomographic images. A positional shift amount derivation unit derives a positional shift amount between the plurality of projection images with the feature point as a reference on a corresponding tomographic plane corresponding to the tomographic image in which the feature point is detected. The reconstruction unit reconstructs the plurality of projection images by correcting the positional shift amount to generate a corrected tomographic image.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of PCT InternationalApplication No. PCT/JP2019/038261, filed on Sep. 27, 2019, which claimspriority to Japanese Patent Application No. 2018-182724, filed on Sep.27, 2018. Each of the above applications is hereby expresslyincorporated by reference, in its entirety, into the presentapplication.

BACKGROUND OF THE INVENTION Technical Field

The present disclosure relates to a tomographic image generatingapparatus, a tomographic image generating method, and a tomographicimage generating program that acquire a plurality of projection imagesby imaging a subject at a plurality of radiation source positions togenerate tomographic images from a plurality of projection images.

Related Art

In recent years, in radiation image capturing apparatuses usingradiation such as X-rays and gamma rays, in order to observe an affectedpart in more detail, tomosynthesis imaging has been proposed in whichimaging is performed by moving a radiation source to emit radiation to asubject from a plurality of radiation source positions and a pluralityof projection images acquired by the imaging are added up to generate atomographic image in which a desired tomographic plane is emphasized. Inthe tomosynthesis imaging, a plurality of projection images are acquiredby imaging the subject at a plurality of radiation source positions bymoving the radiation source in parallel to a radiation detector ormoving the radiation source so as to draw a circular or elliptical arcaccording to the characteristics of the imaging apparatus and requiredtomographic images, and the projection images are reconstructed using,for example, a back projection method, such as a simple back projectionmethod or a filtered back projection method, to generate a tomographicimage.

By generating such a tomographic image on a plurality of tomographicplanes of the subject, it is possible to separate structures overlappingeach other in a depth direction in which the tomographic planes arealigned. Therefore, it is possible to find a lesion that has beendifficult to detect in a two-dimensional image acquired by simpleimaging in the related art. The simple imaging is an imaging method foracquiring one two-dimensional image, which is a transmission image of asubject, by emitting radiation to the subject once.

On the other hand, the tomosynthesis imaging has a problem that areconstructed tomographic image is blurred due to the influence of bodymovement of the subject due to the time difference of imaging at theplurality of radiation source positions. In a case where the tomographicimage is blurred as described above, it is difficult to find a lesionsuch as minute calcification, which is useful for early detection ofbreast cancer, particularly in a case where the breast is a subject.

For this reason, a method of correcting body movement in the case ofgenerating a tomographic image from a projection image acquired bytomosynthesis imaging has been proposed. For example, JP2016-064119Adiscloses a method in which a plurality of tomographic plane projectionimages are acquired by projecting the pixel values of a plurality ofprojection images acquired by tomosynthesis imaging onto coordinatepositions on a desired tomographic plane of a subject based on thepositional relationship between the radiation source position and aradiation detector in a case of imaging the plurality of projectionimages while maintaining the pixel values of the plurality of projectionimages, feature points of an edge, the intersection of the edges, andthe corner of the edge are detected in the plurality of tomographicplane projection images, positional shift between the plurality oftomographic plane projection images is corrected such that the detectedfeature points match, and a tomographic image is generated from theplurality of tomographic plane projection images subjected to positionalshift correction.

On the other hand, the projection image acquired by tomosynthesisimaging is acquired by the radiation transmitted through the subject,and thus it is an image in which a plurality of structures in thesubject overlap each other. Therefore, in a case where the position ofthe radiation source changes, the transmission direction of theradiation in the subject changes, and thus the appearance of featurepoints of the edge, the intersection of the edges, and the corner of theedge included in the projection image differs depending on theprojection image. For example, a structure that appears as theintersection of edges in one projection image may appear as a pluralityof edges that do not have an intersection in another projection image.As in the method disclosed in JP2016-064119A, in a case where thefeature point detected in the tomographic plane projection image isused, the correspondence between the feature points cannot be accuratelyobtained, the accuracy of the correction of the positional shift, andthus a high-quality tomographic image may not be acquired.

SUMMARY OF THE INVENTION

The present disclosure has been made in view of the aforementionedcircumstances, and is to make it possible to acquire a high-qualitytomographic image in which the body movement is accurately corrected.

A tomographic image generating apparatus according to an aspect of thepresent disclosure comprises an image acquisition unit that acquires aplurality of projection images corresponding to a plurality of radiationsource positions, the plurality of projection images being generated bycausing an imaging apparatus to perform tomosynthesis imaging in which aradiation source is moved relative to a detection surface of a detectionunit in order to emit radiation to a subject at the plurality ofradiation source positions according to movement of the radiationsource, a reconstruction unit that reconstructs all or a part of theplurality of projection images to generate a tomographic image on eachof a plurality of tomographic planes of the subject, a feature pointdetecting unit that detects at least one feature point from a pluralityof the tomographic images, and a positional shift amount derivation unitthat derives a positional shift amount between the plurality ofprojection images based on body movement of the subject with the featurepoint as a reference on a corresponding tomographic plane correspondingto the tomographic image in which the feature point is detected, inwhich the reconstruction unit reconstructs the plurality of projectionimages by correcting the positional shift amount to generate a correctedtomographic image on at least one tomographic plane of the subject.

The “radiation source is moved relative to the detection unit” includesa case of moving only the radiation source, a case of moving only thedetection unit, and a case of moving both the radiation source and thedetection unit.

“Reconstruct all or a part of the plurality of projection images” meansthat reconstruction may be performed with all of the plurality ofprojection images, or reconstruction may be performed with two or moreprojection images among the plurality of projection images, not all ofthe plurality of projection images.

The tomographic image generating apparatus according the aspect of thepresent disclosure may further comprise a projection unit that projectsthe plurality of projection images on the corresponding tomographicplane based on a positional relationship between the radiation sourceposition and the detection unit in a case of imaging the plurality ofprojection images to acquire a tomographic plane projection imagecorresponding to each of the plurality of projection images, in whichthe positional shift amount derivation unit derives, as the positionalshift amount between the plurality of projection images, a positionalshift amount between a plurality of the tomographic plane projectionimages based on the body movement of the subject with the feature pointas a reference on the corresponding tomographic plane.

In the tomographic image generating apparatus according to the aspect ofthe present disclosure, the positional shift amount derivation unit mayset a local region corresponding to the feature point in the pluralityof tomographic plane projection images, and derive the positional shiftamount based on the local region.

In the tomographic image generating apparatus according to the aspect ofthe present disclosure, the positional shift amount derivation unit mayset a plurality of first local regions including the feature point inthe plurality of tomographic plane projection images, set a second localregion including the feature point in the tomographic image in which thefeature point is detected, derive a positional shift amount of each ofthe plurality of first local regions with respect to the second localregion as a temporary positional shift amount, and derive the positionalshift amount based on a plurality of the temporary positional shiftamounts.

In this case, the positional shift amount derivation unit may derive thetemporary positional shift amount based on a peripheral region of thefeature point in the second local region.

The “local region” is a region including the feature point in thetomographic image or the tomographic plane projection image, and can bea region having any size smaller than the tomographic image or thetomographic plane projection image.

The local region needs to be larger than the range of movement as thebody movement. The body movement may be about 2 mm in a case of beinglarge. Therefore, in a case of the tomographic image or the tomographicplane projection image in which the size of one pixel is 100 μm square,the local region need only be, for example, a region of 50×50 pixels or100×100 pixels around the feature point.

The “peripheral region of the feature point in the local region” means aregion including the feature point in the local region and being smallerthan the local region.

In the tomographic image generating apparatus according to the aspect ofthe present disclosure, the reconstruction unit may reconstruct theplurality of projection images excluding a target projection image whichcorresponds to a target tomographic plane projection image of which thepositional shift amount is to be derived, and generates the plurality oftomographic images as target tomographic images, and the positionalshift amount derivation unit may derive the positional shift amount ofthe target tomographic plane projection image by using the targettomographic images.

In the tomographic image generating apparatus according to the aspect ofthe present disclosure, the feature point detecting unit may detect aplurality of the feature points from the plurality of tomographicimages, the tomographic image generating apparatus may further comprisea focal plane discrimination unit that discriminates whether thecorresponding tomographic plane corresponding to the tomographic imagein which each of the plurality of feature points is detected is a focalplane, and the positional shift amount derivation unit may derive thepositional shift amount on the corresponding tomographic plane which isdiscriminated to be the focal plane.

The tomographic image generating apparatus according to the aspect ofthe present disclosure may further comprise a combining unit thatcombines two or more tomographic images among the plurality oftomographic images to generate a composite two-dimensional image, inwhich the feature point detecting unit detects a two-dimensional featurepoint in the composite two-dimensional image, and detects the featurepoint corresponding to the two-dimensional feature point from theplurality of tomographic images.

In the tomographic image generating apparatus according to the aspect ofthe present disclosure, the reconstruction unit may reconstruct all or apart of the plurality of projection images while correcting thepositional shift amount to generate a plurality of the correctedtomographic images on the plurality of tomographic planes of the subjectas a plurality of new tomographic images, the feature point detectingunit may detect the feature point from the plurality of new tomographicimages, the positional shift amount derivation unit may derive a newpositional shift amount between the plurality of new projection images,and the reconstruction unit may reconstruct the plurality of projectionimages while correcting the new positional shift amount to generate anew corrected tomographic image on at least one tomographic plane of thesubject.

In the tomographic image generating apparatus according to the aspect ofthe present disclosure, the reconstruction unit, the feature pointdetecting unit, and the positional shift amount derivation unit mayrepeat generating of the new tomographic image, detecting of the featurepoint from the new tomographic image, and deriving of the new positionalshift amount until the new positional shift amount converges.

“Repeat until convergence” means to repeat until the positional shiftamount between the plurality of new tomographic plane projection imagesis equal to or smaller than a predetermined threshold.

The tomographic image generating apparatus according to the aspect ofthe present disclosure may further comprise a positional shift amountdetermination unit that performs image quality evaluation for a regionof interest including the feature point in the corrected tomographicimage, and determines whether the derived positional shift amount isappropriate or inappropriate based on a result of the image qualityevaluation.

In the tomographic image generating apparatus according to the aspect ofthe present disclosure, the positional shift amount determination unitmay perform the image quality evaluation for the region of interestincluding the feature point in the tomographic image, compare the resultof the image quality evaluation for the corrected tomographic image witha result of the image quality evaluation for the tomographic image, anddecide the tomographic image with a better result of the image qualityevaluation as a final tomographic image.

The tomographic image generating apparatus according to the aspect ofthe present disclosure may further comprise an evaluation functionderivation unit that derives an evaluation function for performing imagequality evaluation for a region of interest including the feature pointin the corrected tomographic image, in which the positional shift amountderivation unit derives the positional shift amount for optimizing theevaluation function.

In the tomographic image generating apparatus according to the aspect ofthe present disclosure, the subject may be a breast.

In the tomographic image generating apparatus according the aspect ofthe present disclosure, the positional shift amount derivation unit maychange a search range in a case of deriving the positional shift amountdepending on at least one of a density of a mammary gland, a size of thebreast, an imaging time of the tomosynthesis imaging, a compressionpressure of the breast in a case of the tomosynthesis imaging, or animaging direction of the breast.

A tomographic image generating method according to another aspect of thepresent disclosure comprises acquiring a plurality of projection imagescorresponding to a plurality of radiation source positions, theplurality of projection images being generated by causing an imagingapparatus to perform tomosynthesis imaging in which a radiation sourceis moved relative to a detection surface of a detection unit in order toemit radiation to a subject at the plurality of radiation sourcepositions according to movement of the radiation source, reconstructingall or a part of the plurality of projection images to generate atomographic image on each of a plurality of tomographic planes of thesubject, detecting at least one feature point from a plurality of thetomographic images, deriving a positional shift amount between theplurality of projection images based on body movement of the subjectwith the feature point as a reference on a corresponding tomographicplane corresponding to the tomographic image in which the feature pointis detected, and reconstructing the plurality of projection images bycorrecting the positional shift amount to generate a correctedtomographic image on at least one tomographic plane of the subject.

A program causing a computer to execute the tomographic image generatingmethod according to the aspect of the present disclosure may beprovided.

A tomographic image generating apparatus according to still anotheraspect of the present disclosure comprises a memory that stores acommand to be executed by a computer, and a processor configured toexecute the stored command, in which processor executes processing ofacquiring a plurality of projection images corresponding to a pluralityof radiation source positions, the plurality of projection images beinggenerated by causing an imaging apparatus to perform tomosynthesisimaging in which a radiation source is moved relative to a detectionsurface of a detection unit in order to emit radiation to a subject atthe plurality of radiation source positions according to movement of theradiation source, reconstructing all or a part of the plurality ofprojection images to generate a tomographic image on each of a pluralityof tomographic planes of the subject, detecting at least one featurepoint from a plurality of the tomographic images, deriving a positionalshift amount between the plurality of projection images based on bodymovement of the subject with the feature point as a reference on acorresponding tomographic plane corresponding to the tomographic imagein which the feature point is detected, and reconstructing the pluralityof projection images by correcting the positional shift amount togenerate a corrected tomographic image on at least one tomographic planeof the subject.

According to the present disclosure, it possible to acquire ahigh-quality tomographic image in which the body movement is accuratelycorrected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram of a radiation imagecapturing apparatus to which a tomographic image generating apparatusaccording to a first embodiment of the present disclosure is applied.

FIG. 2 is a diagram of the radiation image capturing apparatus as viewedfrom the direction of arrow A in FIG. 1.

FIG. 3 is a diagram showing a schematic configuration of the tomographicimage generating apparatus realized by installing a tomographic imagegenerating program in a computer in the first embodiment.

FIG. 4 is a diagram illustrating the acquisition of a projection image.

FIG. 5 is a diagram illustrating the generation of a tomographic image.

FIG. 6 is a diagram illustrating the detection of feature points fromthe tomographic image.

FIG. 7 is a diagram illustrating the generation of a tomographic planeprojection image.

FIG. 8 is a diagram illustrating the interpolation of pixel values ofthe tomographic image.

FIG. 9 is a diagram illustrating the setting of a region of interest.

FIG. 10 is a diagram showing the region of interest set in thetomographic plane projection image.

FIG. 11 is a diagram showing an image in the region of interest in acase where no body movement occurs in the first embodiment.

FIG. 12 is a diagram showing an image in the region of interest in acase where body movement occurs in the first embodiment.

FIG. 13 is a diagram illustrating a search range of the region ofinterest.

FIG. 14 is a diagram showing the feature points in a three-dimensionalspace.

FIG. 15 is a diagram showing a display screen for a correctedtomographic image.

FIG. 16 is a flowchart showing a process performed in the firstembodiment.

FIG. 17 is a diagram showing an image in a region of interest in a casewhere no body movement occurs in a second embodiment.

FIG. 18 is a diagram showing an image in the region of interest in acase where body movement occurs in the second embodiment.

FIG. 19 is a diagram illustrating a peripheral region of the featurepoint.

FIG. 20 is a diagram schematically showing a process performed in athird embodiment.

FIG. 21 is a diagram showing a schematic configuration of thetomographic image generating apparatus realized by installing atomographic image generating program in a computer in a fourthembodiment.

FIG. 22 is a diagram illustrating the generation of a feature point map.

FIG. 23 is a flowchart showing a process performed in a fifthembodiment.

FIG. 24 is a diagram showing a warning display.

FIG. 25 is a diagram showing a schematic configuration of thetomographic image generating apparatus realized by installing atomographic image generating program in a computer in a sixthembodiment.

FIG. 26 is a diagram illustrating a ripple artifact.

FIG. 27 is a diagram illustrating the derivation of correspondencepoints.

FIG. 28 is a diagram showing a result of plotting pixel values of thefeature points and the correspondence points.

FIG. 29 is a flowchart showing a process performed in the sixthembodiment.

FIG. 30 is a diagram showing a schematic configuration of thetomographic image generating apparatus realized by installing atomographic image generating program in a computer in a seventhembodiment.

FIG. 31 is a diagram illustrating the setting of a region of interest inthe seventh embodiment.

FIG. 32 is a flowchart showing a process performed in the seventhembodiment.

FIG. 33 is a diagram showing a schematic configuration of thetomographic image generating apparatus realized by installing atomographic image generating program in a computer in an eighthembodiment.

DETAILED DESCRIPTION

Hereinafter, the embodiments of the present disclosure will be describedwith reference to the diagrams. FIG. 1 is a schematic configurationdiagram of a radiation image capturing apparatus to which a tomographicimage generating apparatus according to a first embodiment of thepresent disclosure is applied, and FIG. 2 is a diagram of the radiationimage capturing apparatus as viewed from the direction of arrow A inFIG. 1. A radiation image capturing apparatus 1 is a mammography imagingapparatus that acquires a plurality of radiation images, that is, aplurality of projection images, by imaging a breast M, which is asubject, from a plurality of radiation source positions in order togenerate a tomographic image by performing tomosynthesis imaging of thebreast. As shown in FIG. 1, the radiation image capturing apparatus 1comprises an imaging unit 10, a computer 2 connected to the imaging unit10, and a display unit 3 and an input unit 4 which are connected to thecomputer 2.

The imaging unit 10 comprises an arm unit 12 connected to a base (notshown) by a rotary shaft 11. An imaging table 13 is attached to one endportion of the arm unit 12, and a radiation emission unit 14 is attachedto the other end portion so as to face the imaging table 13. The armunit 12 is configured so that only the end portion to which theradiation emission unit 14 is attached can rotate. Therefore, it ispossible to rotate only the radiation emission unit 14 with the imagingtable 13 fixed. The rotation of the arm unit 12 is controlled by thecomputer 2.

The imaging table 13 comprises a radiation detector 15 such as a flatpanel detector therein. The radiation detector 15 has a detectionsurface 15A of radiation such as X-rays. In addition, a circuit board onwhich a charge amplifier for converting a charge signal read from theradiation detector 15 into a voltage signal, a correlated doublesampling circuit for sampling the voltage signal output from the chargeamplifier, an analog digital (AD) conversion unit for converting thevoltage signal into a digital signal, and the like are provided isprovided inside the imaging table 13. The radiation detector 15corresponds to a detection unit. Although the radiation detector 15 isused as the detection unit in the present embodiment, the detection unitis not limited to the radiation detector 15 as long as radiation can bedetected and converted into an image.

The radiation detector 15 can perform recording and reading of aradiation image repeatedly. A so-called direct-type radiation detectorthat directly converts radiation, such as X-rays, into electric chargesmay be used, or a so-called indirect-type radiation detector thatconverts radiation into visible light and then converts the visiblelight into a charge signal may be used. As a method of reading aradiation image signal, it is desirable to use a so-called thin filmtransistor (TFT) reading method in which a radiation image signal isread by ON and OFF of a TFT switch, or a so-called optical readingmethod in which a radiation image signal is read by emission of readinglight. However, other methods may also be used without being limited tothe above methods.

An X-ray source 16 that is a radiation source is housed inside theradiation emission unit 14. The timing of emission of X-ray that isradiation from the X-ray source 16, and an X-ray generation condition inthe X-ray source 16, that is, selection of target and filter materials,a tube voltage, an emission time, and the like are controlled by thecomputer 2.

The arm unit 12 includes compression plate 17 disposed above the imagingtable 13 to compress the breast M, a support unit 18 that supports thecompression plate 17, and a moving mechanism 19 that moves the supportunit 18 in the vertical direction in FIGS. 1 and 2. Information of thedistance between the compression plate 17 and the imaging table 13, thatis, a compression thickness is input to the computer 2.

The display unit 3 is a display device such as a cathode ray tube (CRT)or a liquid crystal monitor, and displays a message required for theoperation, and the like in addition to a projection image, atwo-dimensional image, and the generated tomographic image acquired asdescribed later. The display unit 3 may include a speaker for outputtingsound.

The input unit 4 includes an input device such as a keyboard, a mouse,or a touch panel system, and receives an operation of the radiationimage capturing apparatus 1 by the operator. In addition, the input unit4 receives an input of various kinds of information such as imagingconditions and the instruction of correction of the information, whichare required to perform the tomosynthesis imaging. In the presentembodiment, each part of the radiation image capturing apparatus 1operates in accordance with the information input from the input unit 4by the operator.

A tomographic image generating program according to the presentembodiment is installed in the computer 2. In the present embodiment,the computer may be a workstation or a personal computer that isdirectly operated by the operator, or may be a server computer connectedto these through a network. The tomographic image generating program isdistributed in a state of being recorded on a recording medium such as adigital versatile disc (DVD) or a compact disc read only memory(CD-ROM), and is installed in the computer from the recording medium.Alternatively, the tomographic image generating program is stored in astorage device of a server computer connected to the network, or in anetwork storage so as to be accessible from the outside, and isdownloaded and installed in the computer as necessary.

FIG. 3 is a diagram showing a schematic configuration of the tomographicimage generating apparatus realized by installing a tomographic imagegenerating program in the computer 2 in the first embodiment. As shownin FIG. 3, the tomographic image generating apparatus comprises acentral processing unit (CPU) 21, a memory 22, and a storage 23 as theconfiguration of a standard computer.

The storage 23 includes a storage device such as a hard disk drive or asolid state drive (SSD), and stores various kinds of informationincluding a program for driving each unit of the radiation imagecapturing apparatus 1 and the tomographic image generating program. Inaddition, the storage 23 also stores the projection image acquired bytomosynthesis imaging, and the tomographic image and the tomographicplane projection image generated as described later.

The memory 22 temporarily stores programs and the like stored in thestorage 23 so that the CPU 21 executes various kinds of processing. Thetomographic image generating program defines, as the processing to beexecuted by the CPU 21, image acquisition processing of acquiring aplurality of projection images of the breast M corresponding to aplurality of radiation source positions by causing the radiation imagecapturing apparatus 1 to perform tomosynthesis imaging, reconstructionprocessing of reconstructing all or a part of the plurality ofprojection images to generate a tomographic image on each of a pluralityof tomographic planes of the breast M which is the subject, featurepoint detecting processing of detecting at least one feature point froma plurality of the tomographic images, projection processing ofprojecting the plurality of projection images on the correspondingtomographic plane corresponding to the tomographic image in which thefeature point is detected, based on a positional relationship betweenthe position of the X-ray source 16 and the radiation detector 15 in acase of imaging the plurality of projection images, and acquiring atomographic plane projection image corresponding to each of theplurality of projection images, positional shift amount derivationprocessing of deriving a positional shift amount between the pluralityof tomographic plane projection images based on the body movement of thebreast M with the feature point as a reference on a correspondingtomographic plane, reconstruction processing of reconstructing theplurality of projection images by correcting the positional shift amountto generate a corrected tomographic image on at least one tomographicplane of the subject, and display control processing of displaying thetomographic image and the like on the display unit 3.

Then, the CPU 21 executes these kinds of processing according to thetomographic image generating program, so that the computer 2 functionsas an image acquisition unit 31, a reconstruction unit 32, a featurepoint detecting unit 33, a projection unit 34, a positional shift amountderivation unit 35, and a display controller 36.

In the case of performing image acquisition processing, the X-ray source16 is moved by rotating the arm unit 12 around the rotary shaft 11,X-rays are emitted to the breast M as a subject at a plurality ofradiation source positions according to the movement of the X-ray source16 under the predetermined imaging conditions for tomosynthesis imaging,X-rays transmitted through the breast M are detected by the radiationdetector 15, and a plurality of projection images Gi (i=1 to n, where nis the number of radiation source positions; for example, n=15) at aplurality of radiation source positions are acquired by the imageacquisition unit 31.

FIG. 4 is a diagram illustrating the acquisition of the projection imageGi. As shown in FIG. 4, the X-ray source 16 is moved to each radiationsource position of S1, S2, . . . , Sn, the X-ray source 16 is driven ateach radiation source position to irradiate the breast M with X-ray, andthe X-rays transmitted through the breast M are detected by theradiation detector 15. As a result, the projection images G1, G2, . . ., Gn are acquired corresponding to the radiation source positions S1 toSn. At each of the radiation source positions S1 to Sn, X-rays of thesame dose are emitted to the breast M. The plurality of acquiredprojection images Gi are stored in the storage 23. The plurality ofprojection images Gi may be acquired by a program separate from thetomographic image generating program and stored in the storage 23 or theexternal storage device. In this case, the image acquisition unit 31reads the plurality of projection images Gi stored in the storage 23 orthe external storage device from the storage 23 or the external storagedevice for reconstruction processing and the like.

In FIG. 4, the radiation source position Sc is a radiation sourceposition where an optical axis X0 of the X-rays emitted from the X-raysource 16 is perpendicular to the detection surface 15A of the radiationdetector 15. The radiation source position Sc is referred to as areference radiation source position Sc, and the projection image Gcacquired by irradiating the breast M with X-rays at the referenceradiation source position Sc is referred to as a reference projectionimage Gc. Here, “the optical axis X0 of the X-ray is perpendicular tothe detection surface 15A of the radiation detector 15” means that theoptical axis X0 of the X-ray crosses the detection surface 15A of theradiation detector 15 at an angle of 90°. However, without being limitedto this, a case where the optical axis X0 of the X-rays crosses thedetection surface 15A of the radiation detector 15 with a certain degreeof error with respect to 90° may be included. For example, a case wherethe optical axis X0 of the X-ray crosses the detection surface 15A ofthe radiation detector 15 with an error of about ±3° with respect to 90°is included in “the optical axis X0 of the X-ray is perpendicular to thedetection surface 15A of the radiation detector 15” in the presentembodiment.

The reconstruction unit 32 reconstructs all or a part of the pluralityof projection images Gi to generate the tomographic image in which adesired tomographic plane of the breast M is emphasized. Specifically,the reconstruction unit 32 reconstructs all or a part of the pluralityof projection images Gi by a well-known back projection method such as asimple back projection method or a filtered back projection method togenerate a plurality of tomographic images Dj (j=1 to m) on each of aplurality of tomographic planes of the breast M, as shown in FIG. 5. Inthis case, a three-dimensional coordinate position in athree-dimensional space including the breast M is set, pixel values ofcorresponding pixel positions of the plurality of projection images Giare reconstructed for the set three-dimensional coordinate position, andthe pixel value of the coordinate position is calculated. In a casewhere the positional shift amount based on body movement of the breast Min the tomosynthesis imaging is derived as described later, thereconstruction unit 32 corrects the positional shift amount andreconstructs the plurality of projection images Gi to generate acorrected tomographic image in which body movement is corrected.

The feature point detecting unit 33 detects at least one feature pointfrom a plurality of the tomographic images Dj. FIG. 6 is a diagramillustrating the detection of the feature points. Here, the detection ofthe feature points from one tomographic image Dk among the plurality oftomographic images Dj will be described. As shown in FIG. 6, thetomographic image Dk includes point-like structures E1 to E3 such ascalcification, and intersections E4 and E5 of edges such asintersections of blood vessels on the tomographic plane of the breast Min which the tomographic image Dk is acquired.

The feature point detecting unit 33 detects the point-like structure,such as calcification, as a feature point from the tomographic image Dkby using an algorithm of known computer aided diagnosis (hereinafter,referred to as CAD). In addition, edges, intersections of edges, cornersof edges, and the like included in the tomographic image Dk are detectedas feature points by using an algorithm such as a Harris's cornerdetection method, a scale-invariant feature transform (SIFT), featuresfrom accelerated segment test (FAST), or speeded up robust features(SURF). For example, the feature point detecting unit 33 detects apoint-like structure E1 included in the tomographic image Dk shown inFIG. 6 as a feature point F1.

Here, for the sake of explanation, only one feature point F1 is detectedfrom one tomographic image Dk, but it is preferable to detect aplurality of feature points. For example, all of point-like structuresE1 to E3 and intersections E4 and E5 included in the tomographic imageDk shown in FIG. 6 may be detected as the feature points. The featurepoint may be only one pixel in the tomographic image Dk, or may be aplurality of pixels indicating the positions of feature structures.Also, for the sake of explanation, the feature point is detected onlyfrom one tomographic image Dk, but it is assumed that a plurality offeature points are actually detected from each of the plurality oftomographic images.

The projection unit 34 projects the plurality of projection images Gi onthe corresponding tomographic plane which is the tomographic planecorresponding to the tomographic image in which the feature point F1 isdetected, based on the positional relationship between the radiationsource position and the radiation detector 15 in a case of imaging theplurality of projection images Gi. As a result, the projection unit 34acquires the tomographic plane projection image GTi corresponding toeach of the plurality of projection images Gi. Hereinafter, theacquisition of the tomographic plane projection image GTi will bedescribed. In the present embodiment, since the feature points aredetected in each of the plurality of tomographic images Dj, theplurality of projection images Gi are projected on the plurality oftomographic planes Tj corresponding to plurality of tomographic imagesDj to generate the tomographic plane projection image GTi.

FIG. 7 is a diagram illustrating the projection of the projection image.In FIG. 7, a case will be described in which one projection image Giacquired at the radiation source position Si is projected on onetomographic plane Tj of the breast M. In the present embodiment, asshown in FIG. 7, at the intersecting position of a straight lineconnecting the radiation source position Si and the pixel position onthe projection image Gi, and the tomographic plane Tj, the pixel valueof the projection image Gi positioned on the straight line is projected.

The tomographic image generated on the projection image Gi and thetomographic plane Tj is composed of a plurality of pixels discretelyarranged two-dimensionally at a predetermined sampling interval, andpixels are arranged in grid points having a predetermined samplinginterval. In FIG. 7, a short line segment orthogonal to the projectionimage Gi and the tomographic plane Tj indicates the pixel divisionposition. Therefore, in FIG. 7, the center position of the pixeldivision position is the pixel position which is the grid point.

Here, the relationship of the coordinates (sxi, syi, szi) of theradiation source position at the radiation source position Si, thecoordinates (pxi, pyi) of the pixel position Pi in the projection imageGi, and the coordinates (tx, ty, tz) of the projection position on thetomographic plane Tj is expressed by Equation (1) below. In the presentembodiment, a z-axis is set to a direction orthogonal to the detectionsurface 15A of the radiation detector 15, a y-axis is set to a directionparallel to a direction in which the X-ray source 16 moves in thedetection surface of the radiation detector 15, and an x-axis is set toa direction orthogonal to the y-axis.

pxi=(tx×szi−sxi×tz)/(szi−tz)

pyi=(ty×szi−syi×tz)/(szi−tz)  (1)

Therefore, by setting pxi and pyi in Equation (1) to the pixel positionof the projection image Gi, and solving Equation (1) for tx and ty, theprojection position on the tomographic plane Tj on which the pixel valueof the projection image Gi is projected can be calculated. Therefore, byprojecting the pixel value of the projection image Gi on the projectionposition on the calculated tomographic plane Tj, the tomographic planeprojection image GTi is generated.

In this case, the intersection point of the straight line connecting theradiation source position Si and the pixel position on the projectionimage Gi, and the tomographic plane Tj may not be positioned on thepixel position on the tomographic plane Tj. For example, as shown inFIG. 8, the projection position (tx, ty, tz) on the tomographic plane Tjmay be positioned between the pixel positions O1 to O4 of thetomographic image Dj on the tomographic plane Tj. In this case, thepixel value of each pixel position need only be calculated by performingan interpolation calculation using the pixel value of the projectionimage at the plurality of projection positions around the pixelpositions O1 to O4. As the interpolation calculation, a linearinterpolation calculation that weights the pixel value of the projectionimage at the projection position according to the distance between thepixel position and the plurality of projection positions around thepixel position can be used. In addition, any method such as a non-linearbicubic interpolation calculation using more pixel values of projectionpositions around the pixel position and a B-spline interpolationcalculation can be used. Also, in addition to the interpolationcalculation, the pixel value at the projection position closest to thepixel position may be used as the pixel value at the pixel position. Asa result, for the projection image Gi, the pixel values at all of thepixel positions of the tomographic plane Tj can be obtained. In thepresent embodiment, for each of the plurality of projection images Gi,the tomographic plane projection image GTi having pixel values obtainedat all of the pixel positions of the tomographic plane Tj is generated.Therefore, in one tomographic plane, the number of tomographic planeprojection images GTi matches the number of projection images Gi.

The positional shift amount derivation unit 35 derives the positionalshift amount between the plurality of tomographic plane projectionimages GTi based on the body movement of the breast M during thetomosynthesis imaging. First, the positional shift amount derivationunit 35 sets the local region corresponding to the feature point F1 as aregion of interest for the plurality of tomographic plane projectionimages GTi. Specifically, the local region having a predetermined sizecentered on the coordinate position of the feature point F1 is set asthe region of interest. FIG. 9 is a diagram illustrating the setting ofa region of interest. In FIG. 9, for the sake of explanation, it isassumed that three projection images G1 to G3 are projected on thetomographic plane Tj to generate the tomographic plane projection imagesGT1 to GT3. As shown in FIG. 9, the positional shift amount derivationunit 35 sets the region of interest Rf0 centered on the coordinateposition of the feature point F1 in the tomographic image Dj on thetomographic plane Tj. The regions of interest R1 to R3 corresponding tothe region of interest Rf0 are set in the tomographic plane projectionimages GT1 to GT3. The broken line in FIG. 9 indicates the boundarybetween the regions of interest R1 to R3 and the other regions.Therefore, the positions of the region of interest Rf0 and the regionsof interest R1 to R3 coincide with each other on the tomographic planeTj. FIG. 10 is a diagram showing the regions of interest R1 to R3 set inthe tomographic plane projection images GT1 to GT3. The body movementmay be about 2 mm in a case of being large. Therefore, in a case of thetomographic image or the tomographic plane projection image in which thesize of one pixel is 100 μm square, the regions of interest R1 to R3need only be, for example, a region of 50×50 pixels or 100×100 pixelsaround the feature point F1.

Further, the positional shift amount derivation unit 35 performsregistration of the regions of interest R1 to R3. At this time, theregistration is performed with reference to the region of interest setin the reference tomographic plane projection image. In the presentembodiment, the registration of other regions of interest is performedwith reference to the region of interest set in the tomographic planeprojection image (reference tomographic plane projection image) forreference projection image (referred to as Gs) acquired at the radiationsource position Sc in which the optical axis X0 of the X-rays from theX-ray source 16 is orthogonal to the radiation detector 15.

Here, it is assumed that the region of interest R2 shown in FIG. 10 isset in the reference tomographic plane projection image. In this case,the positional shift amount derivation unit 35 performs the registrationof the regions of interest R1 and R3 with respect to the region ofinterest R2, and derives the shift vector representing the movementdirection and the movement amount of the regions of interest R1 and R3with respect to the region of interest R2 as the positional shiftamount. The registration means that the movement direction and themovement amount of the regions of interest R1 and R3 with respect to theregion of interest R2 are obtained in a predetermined search range suchthat the correlation between the regions of interest R1 and R3, and theregion of interest R2. Here, the normalized cross correlation may beused as the correlation. Further, since the reference tomographic planeprojection image of one of the tomographic plane projection images GTiis used as a reference, the shift vector is one less than the number oftomographic plane projection images. For example, in a case where thenumber of tomographic plane projection images is 15, the number of shiftvectors is 14. In a case where the number of tomographic planeprojection images is 3, the number of shift vectors is 2.

FIG. 11 is a diagram showing the image of three regions of interest R1to R3 in a case where no body movement occurs during acquisition of theprojection images G1 to G3. In FIG. 11, the center position of theregions of interest R1 to R3, that is, the positions P1 to P3corresponding to the feature points F1 in the tomographic planeprojection images GT1 to GT3 are shown, and images F2 of the featurepoints F1 included in the regions of interest R1 to R3 are indicated bya large circle. As shown in FIG. 11, in a case where no body movementoccurs during the acquisition of the projection images G1 to G3, in allof three regions of interest R1 to R3, the positions P1 to P3corresponding to the feature points F1 and the positions of the imagesF2 of the feature point s F1 match each other. Therefore, the shiftvectors of the regions of interest R1 and R3 with respect to the regionof interest R2, that is, the positional shift amounts are all 0.

FIG. 12 is a diagram showing the image of three regions of interest R1to R3 in a case where body movement occurs during acquisition of theprojection images G2 and G3 among the projection images G1 to G3. InFIG. 12, since no body movement occurs during the acquisition of theprojection image G1 and the projection image G2, the positions P1 and P2corresponding to the feature point F1 in the regions of interest R1 andR2 and the position of the image F2 of the feature point F1 included inthe regions of interest R1 and R2 match each other. For this reason, thepositional shift amount of the region of interest R1 with respect to theregion of interest R2 is 0. On the other hand, since no body movementoccurs during the acquisition of the projection image G2 and theprojection image G3, the position P3 corresponding to the feature pointF1 in the region of interest R3 and the position of the image F2 of thefeature point F1 included in the region of interest R3 match each other.Therefore, due to the movement amount and the movement direction of theregion of interest R3 with respect to the region of interest R2, theshift vector V10 having a size and a direction is derived as thepositional shift amount.

In a case where the positional shift amount is derived, a search rangein a case of deriving the positional shift amount may be changeddepending on at least one of a density of a mammary gland for the breastM, a size of the breast M, an imaging time of the tomosynthesis imaging,a compression pressure of the breast M in a case of the tomosynthesisimaging, or an imaging direction of the breast. FIG. 13 is a diagramillustrating the change of the search range. As shown in FIG. 13, twotypes of search ranges, a small search range H1 and a large search rangeH2, are set as the search ranges of the regions of interest R1 and R3with respect to the region of interest R2 which is a reference.

Here, in a case where the density of the mammary gland is small, theamount of fat in the breast M is large, and thus the body movement tendsto be large in a case of imaging. Also, in a case where the breast M islarge, the body movement tends to be large in a case of imaging. Inaddition, as the tomosynthesis imaging time is longer, the body movementduring imaging tends to be large. Further, in a case where the imagingdirection of the breast M is a medio-lateral oblique (MLO) direction,the body movement in a case of imaging tends to be large than acranio-caudal (CC) direction.

Therefore, it is preferable that the positional shift amount derivationunit 35 changes a search range in a case of deriving the positionalshift amount by receiving, from the input unit 4, the input of at leastone information of a density of a mammary gland for the breast M, a sizeof the breast M, an imaging time of the tomosynthesis imaging, acompression pressure of the breast M in a case of the tomosynthesisimaging, or an imaging direction of the breast M. Specifically, in acase where the body movement tends to increase, the large search rangeH2 shown in FIG. 13 need only be set. On the contrary, in a case wherethe body movement tends to increase, the small search range H1 shown inFIG. 13 need only be set.

In the above, for the sake of explanation, the positional shift amountbetween the plurality of tomographic plane projection image GTi isderived for one feature point F1 is detected on one tomographic planeTj. In practice, however, as shown in FIG. 14, the positional shiftamount derivation unit 35 derives a positional shift amount for aplurality of different feature points F (here, ten feature points shownby black circles) in a three-dimensional space in the breast M expressedby the plurality of tomographic images Dj. As a result, for thetomographic plane projection image corresponding to the projection imageacquired in a state in which body movement occurs, positional shiftamounts for a plurality of different feature points F are derived. Thepositional shift amount derivation unit 35 interpolates the positionalshift amounts for the plurality of different feature points F withrespect to the coordinate positions of the three-dimensional space forgenerating the tomographic image Dj. As a result, for the tomographicplane projection image acquired in a state in which body movementoccurs, the positional shift amount derivation unit 35 derives thepositional shift amounts in a case of performing reconstruction for allof the coordinate positions of the three-dimensional space forgenerating a tomographic image.

The reconstruction unit 32 reconstructs the projection image Gi whilecorrecting the derived positional shift amount to generate the correctedtomographic image Dhj in which the body movement is corrected.Specifically, in a case where the back projection method is used forreconstruction, the pixel of the projection image Gi in which thepositional shift occurs is reconstructed by correcting the positionalshift such that the pixel corresponding to the other projection image isprojected on the position to be back projected, based on the derivedpositional shift amount.

Instead of deriving the positional shift amount at the plurality ofdifferent feature points F, one positional shift amount may be derivedfrom the plurality of different feature points F. In this case, theregion of interest is set for each of the plurality of different featurepoints F, and the positional shift amount is derived on the assumptionthat the entire region of interest moves in the same direction by thesame amount. In this case, the positional shift amount need only bederived such that the representative values (for example, mean value,median value, or maximum value) of the correlation for all of theregions of interest between the tomographic plane projection images thatare target of positional shift amount derivation are maximized. Here, ina case where the signal-to-noise ratio of each feature point F in thetomographic plane projection image is not very good, the accuracy ofderiving the positional shift amount deteriorates. However, by derivingone positional shift amount from the plurality of different featurepoints F in this way, even in a case where the signal-to-noise ratio ofeach feature point F is not very good, the accuracy of deriving thepositional shift amount can be improved.

The three-dimensional space in the breast M represented by the pluralityof tomographic images Dj may be divided into a plurality ofthree-dimensional regions, and one positional shift amount may bederived from the plurality of feature points F in the same manner asdescribed above for each region.

The display controller 36 displays the generated corrected tomographicimage on the display unit 3. FIG. 15 is a diagram showing the displayscreen of the corrected tomographic image. As shown in FIG. 15, thetomographic image Dj before body movement correction and the correctedtomographic image Dhj subjected to body movement correction aredisplayed on a display screen 40. A label 41 of “before correction” isgiven to the tomographic image Dj so that it can be seen that the bodymovement is not corrected. A label 42 of “after correction” is given tothe corrected tomographic image Dhj such that it can be seen that bodymovement is corrected. The label 41 may be given only to the tomographicimage Dj, or the label 42 may be given only to the corrected tomographicimage Dhj. It is needless to say that only the corrected tomographicimage Dhj may be displayed. In FIG. 15, a broken line indicates that thestructures included in the tomographic image Dj before correction isblurred, and a solid line indicates that the structures included in thecorrected tomographic image Dhj is not blurred.

Further, it is preferable that the tomographic image Dj and thecorrected tomographic image Dhj display the same cross section. In acase of switching the tomographic plane to be displayed according to theinstruction from the input unit 4, it is preferable to link thetomographic plane to be displayed in the tomographic image Dj and thecorrected tomographic image Dhj. In addition to the tomographic image Djand the corrected tomographic image Dhj, the projection image Gi may bedisplayed.

The operator can confirm the success or failure of the body movementcorrection by looking at the display screen 40. Further, in a case wherethe body movement is too large, even in a case where the tomographicimage is generated by performing reconstruction while correcting thepositional shift amount as in the present embodiment, the body movementcannot be corrected accurately, and the body movement correction mayfail. In such a case, the tomographic image Dj may have a higher imagequality than the corrected tomographic image Dhj due to the failure ofthe body movement correction. Therefore, the input unit 4 may receive aninstruction to store any of the tomographic image Dj or the correctedtomographic image Dhj, and the instructed image may be stored in thestorage 23 or the external storage device.

Next, the processing performed in the first embodiment will bedescribed. FIG. 16 is a flowchart showing a process performed in thefirst embodiment. In a case where the instruction of an operator tostart the processing is received through the input unit 4, the imageacquisition unit 31 causes the radiation image capturing apparatus 1 toperform the tomosynthesis imaging to acquire a plurality of projectionimages Gi (step ST1). Then, the reconstruction unit 32 reconstructs allor a part of the plurality of projection images Gi to generate aplurality of tomographic images Dj (step ST2). Then, the feature pointdetecting unit 33 detects at least one feature point from a plurality ofthe tomographic images Dj (step ST3). The projection unit 34 projectsthe plurality of projection images Gi on the corresponding tomographicplane corresponding to the tomographic image in which the feature pointF1 is detected, based on the positional relationship between theradiation source position and the radiation detector 15 in a case ofimaging the plurality of projection images Gi, and acquires thetomographic plane projection image GTi corresponding to each of theplurality of projection images Gi (step ST4).

Next, the positional shift amount derivation unit 35 derives thepositional shift amount between the plurality of tomographic planeprojection image GTi (step ST5). Further, the reconstruction unit 32reconstructs the plurality of projection images Gi while correcting thepositional shift amount, and thereby generates a corrected tomographicimage Dhj (step ST6). Then, the display controller 36 displays thecorrected tomographic image Dhj on the display unit 3 (step ST7), andthe processing is terminated. The generated corrected tomographic imageDhj is transmitted to the external storage device (not shown) andstored.

As described above, according to the first embodiment, the plurality ofprojection images Gi by tomosynthesis imaging are acquired, all or apart of the plurality of projection images Gi are reconstructed, and thetomographic image Dj of each of the plurality of tomographic plane Tj ofthe breast M are generated. At least one feature point is detected fromthe plurality of tomographic images Dj, the plurality of projectionimages Gi are projected on the corresponding tomographic planecorresponding to the tomographic image in which the feature point isdetected, based on the positional relationship between the radiationsource position and the radiation detector 15 in a case of imaging theplurality of projection images Gi, and the tomographic plane projectionimage GTi corresponding to each of the plurality of projection images Giare acquired. Further, on the corresponding tomographic plane, thepositional shift amount between the plurality of tomographic planeprojection images is derived with the feature point as a reference, andthe plurality of projection images Gi are reconstructed by correctingthe positional shift amount to generate the corrected tomographic imageDhj.

As described above, in the first embodiment, the feature points aredetected from the plurality of tomographic images Dj, not from theprojection image Gi or the tomographic plane projection image GTi. Here,the tomographic image Dj includes only the structures included on thecorresponding tomographic plane Tj. Therefore, the structures on othertomographic planes included in the projection image Gi are not includedin the tomographic image Dj. Therefore, according to the firstembodiment, the feature points can be detected accurately without beingaffected by the structures of other tomographic planes. Therefore, thepositional shift amount between the plurality of projection images Gican be appropriately derived, and as a result, according to the presentembodiment, a high-quality corrected tomographic image Dhj with reducedeffects of body movement can be acquired.

Hereinafter, the second embodiment of the present disclosure will bedescribed. The configuration of the tomographic image generatingapparatus according to the second embodiment is the same as theconfiguration of the tomographic image generating apparatus according tothe first embodiment shown in FIG. 3, only the processing to beperformed is different, and thus the detailed description of theapparatus is omitted. In the first embodiment, the positional shiftamount is derived between the tomographic plane projection images GTi.In the second embodiment, the region of interest Rf0 centered on thecoordinate position of the feature point F1 is set in the tomographicimage Dj, and the positional shift amount of the region of interest Riset in the tomographic plane projection image GTi with respect to theset region of interest Rf0 is derived as a temporary positional shiftamount. Then, the difference from the first embodiment is that thepositional shift amount between the plurality of tomographic planeprojection images GTi is derived based on the derived amount oftemporary positional shift amount. The region of interest Ri set in theplurality of tomographic plane projection images GTi corresponds to thefirst local region, and the region of interest Rf0 set in thetomographic image Dj corresponds to the second local region.

FIG. 17 is a diagram illustrating the derivation of the positional shiftamount in the second embodiment. The region of interest Rf0 and theregions of interest R1 to R3 in FIG. 17 are the same as the region ofinterest Rf0 and the regions of interest R1 to R3 shown in FIG. 9. Inthe second embodiment, first, the positional shift amount derivationunit 35, as reference to the region of interest Rf0 set in thetomographic image Dj, derives the positional shift amounts of theregions of interest R1 to R3 set in the tomographic plane projectionimage GTi (GT1 to GT3 in FIG. 17) with respect to the region of interestRf0 as a temporary positional shift amount. In a case where no bodymovement occurs during the acquisition of the projection images G1 toG3, in all of three regions of interest R1 to R3, the positions P1 to P3corresponding to the feature points F1 and the positions of the imagesF2 of the feature point s F1 match each other. Therefore, the shiftvectors (hereinafter, referred to as Vf1, Vf2, and Vf3) of the regionsof interest R1 to R3 with respect to the region of interest Rf0, thatis, the temporary positional shift amounts are all 0.

FIG. 18 is a diagram showing the image of three regions of interest R1to R3 in a case where body movement occurs during acquisition of theprojection images G2 and G3 among the projection images G1 to G3. InFIG. 18, since no body movement occurs during the acquisition of theprojection image G1 and the projection image G2, the positions P1 and P2corresponding to the feature point F1 in the regions of interest R1 andR2 and the position of the image F2 of the feature point F1 included inthe regions of interest R1 and R2 match each other. For this reason, thepositional shift amount of the region of interest R1 and R2 with respectto the region of interest Rf0 is 0. On the other hand, since no bodymovement occurs during the acquisition of the projection image G2 andthe projection image G3, the position P3 corresponding to the featurepoint F1 in the region of interest R3 and the position of the image F2of the feature point F1 included in the region of interest R3 match eachother. Therefore, the movement amount and the movement direction of theregion of interest R3 are generated with respect to the region ofinterest Rf0. Therefore, the shift vectors Vf1 and Vf2 of the regions ofinterest R1 and R2 with respect to the region of interest Rf0, that is,the temporary positional shift amount is 0, but the shift vector Vf3 ofthe region of interest R3 with respect to the region of interest Rf0,that is, the temporary positional shift amount has a value.

In the second embodiment, the positional shift amount derivation unit 35derives the positional shift amount between the tomographic planeprojection images GTi based on the temporary positional shift amount.Specifically, as in the first embodiment, the positional shift amount isderived with reference to the projection image acquired at the referenceradiation source position Sc in which the optical axis X0 of the X-raysfrom the X-ray source 16 is orthogonal to the radiation detector 15.Here, in a case where the projection image G2 is a reference tomographicplane projection image, the positional shift amount derivation unit 35derives the positional shift amounts of the tomographic plane projectionimage GT1 and the tomographic plane projection image GT2 by thedifference value Vf1−Vf2 of the shift vectors Vf1 and Vf2 of the regionsof interest R1 and R2 with respect to the region of interest Rf0. Also,the positional shift amount derivation unit 35 derives the positionalshift amounts of the tomographic plane projection image GT3 and thetomographic plane projection image GT2 by the difference value Vf3−Vf2of the shift vectors Vf3 and Vf2 of the regions of interest R3 and R2with respect to the region of interest Rf0.

As described above, in the second embodiment, the temporary positionalshift amounts of the regions of interest R1 to R3 set on the tomographicplane projection image GTi with respect to the region of interest Rf0set on the tomographic image Dj are derived, and the positional shiftamount between the tomographic plane projection image GTi is derivedbased on the temporary positional shift amount. Here, since the regionof interest Rf0 is set on the tomographic image Dj, unlike theprojection image Gi, only the structures on the tomographic plane fromwhich the tomographic image Dj is acquired are included. Therefore,according to the second embodiment, the positional shift amount isderived by reducing the influence of the structures included on thetomographic plane other than the tomographic plane in which the featurepoints are set. Therefore, according to the second embodiment, theinfluence of the structures on other tomographic planes can be reduced,the positional shift amount between the plurality of projection imagesGi can be accurately derived, and as a result, according to the secondembodiment, a high-quality corrected tomographic image Dhj with reducedeffects of body movement can be acquired.

In the second embodiment, as in the first embodiment, a search range ina case of deriving the positional shift amount may be changed dependingon at least one of a density of a mammary gland for the breast M, a sizeof the breast M, an imaging time of the tomosynthesis imaging, acompression pressure of the breast M in a case of the tomosynthesisimaging, or an imaging direction of the breast M.

Further, in the second embodiment, the shift vectors Vf1 to Vf3 of theregions of interest R1 to R3 with respect to the region of interest Rf0are derived as a temporary positional shift amount, but in this case,the peripheral region Ra0 that is smaller than the region of interestRf0 may be set around the feature point F1 of the region of interest Rf0as shown in FIG. 19, and the shift vector may be derived based on theperipheral region Ra0. In this case, the shift vector may be derivedusing only the peripheral region Ra0. Further, in a case of deriving thecorrelation between the regions of interest R1 to R3, the peripheralregion Ra0 may be weighted larger than the regions other than theperipheral region Ra0 in the regions of interest R1 to R3.

Further, in the second embodiment, the region of interest Rf0 is set inthe tomographic image Dj, but the tomographic image to be generated maybe different for each tomographic plane projection image GTi from whichthe temporary positional shift amount is derived. Specifically, it ispreferable to generate the tomographic image excluding the targetprojection image corresponding to the target tomographic planeprojection image to be derived from which the temporary positional shiftamount is derived. Hereinafter, this case will be described as the thirdembodiment.

FIG. 20 is a diagram schematically showing a process performed in athird embodiment. Here, a case will be described in which on thetomographic plane Tj of the breast M, the temporary positional shiftamount for the projection image G1 is derived with the projection imageG1 as the target projection image among fifteen projection images G1 toG15, and the tomographic plane projection image GT1 as the targettomographic plane projection image. In this case, the reconstructionunit 32 reconstructs the projection images G2 to G15 excluding theprojection image G1 on the tomographic plane Tj to generate atomographic image (referred to as Dj_1). Then, the feature pointdetecting unit 33 detects the feature point from the tomographic imageDj_1, the projection unit 34 generates the tomographic plane projectionimages GT1 to GT15 from the projection images G1 to G15, and thepositional shift amount derivation unit 35 sets the region of interestRf0_1 on the tomographic image Dj_1, and derives the shift vector Vf1 ofthe region of interest R1 set on the tomographic plane projection imageGT1 with respect to the region of interest Rf0_1 as the temporarypositional shift amount.

In a case of deriving the temporary positional shift amount for theprojection image G2, the reconstruction unit 32 reconstructs theprojection images G1, G3 to G15 excluding the projection image G2 togenerate the tomographic image (referred to as Dj_2). Then, the featurepoint detecting unit 33 detects the feature point from the tomographicimage Dj_2, the projection unit 34 generates the tomographic planeprojection images GT1 to GT15 from the projection images G1 to G15, andthe positional shift amount derivation unit 35 sets the region ofinterest Rf0_2 on the tomographic image Dj_2, and derives the shiftvector Vf2 of the region of interest R2 set on the tomographic planeprojection image GT2 with respect to the region of interest Rf0_2 as thetemporary positional shift amount.

Then, the target tomographic plane projection image is sequentiallychanged to derive the temporary positional shift amount for all of thetomographic plane projection images GTi, and as in the secondembodiment, the positional shift amount between the tomographic planeprojection images GTi is derived based on the temporary positional shiftamount.

As described above, according to the third embodiment, the temporarypositional shift amount is derived using the tomographic image that isnot affected by the target projection image. Accordingly, the temporarypositional shift amount can be more accurately derived, and as a result,the positional shift amount can be accurately derived.

In the third embodiment, the reconstructing of the tomographic imageexcluding the target projection image may be calculated, as shown inEquation (2) below, by subtracting the corresponding pixel value Gp ofthe target projection image from the pixel value Dp of each pixel of thetomographic image Dj generated by reconstructing all of the projectionimage Gi, and multiplying the subtracted pixel value by n/(n−1).Although the method of Equation (2) is a simple method, the amount ofcalculation for generating the tomographic image excluding the targetprojection image can be reduced, and the processing for deriving thetemporary positional shift amount can be performed at high speed.

Tomographic image excluding target projection image=(Dp−Gp)×n/(n−1)  (2)

Hereinafter, the fourth embodiment of the present disclosure will bedescribed. FIG. 21 is a diagram showing a schematic configuration of thetomographic image generating apparatus realized by installing atomographic image generating program in the computer 2 in the fourthembodiment. In FIG. 21, the same reference numbers as those in FIG. 3are assigned to the same configurations as those in FIG. 3, and detaileddescription thereof will be omitted here. The fourth embodiment isdifferent from the first embodiment in that the tomographic imagegenerating apparatus further comprises a combining unit 37 that combinestwo or more tomographic images among the plurality of tomographicimages, or at least one of the plurality of tomographic images and atleast one of the plurality of projection images Gi to generate thecomposite two-dimensional image.

In the fourth embodiment, the combining unit 37 generates a compositetwo-dimensional image by using, for example, the method disclosed inJP2014-128716A. The method disclosed in JP2014-128716A is a method inwhich two or more tomographic images among the plurality of tomographicimages, or at least one of the plurality of tomographic images and atleast one of the plurality of projection images Gi are projected in thedepth direction in which the tomographic planes of the subject arearranged to generate the composite two-dimensional image. The method ofgenerating the composite two-dimensional image is not limited thereto.For example, the minimum value projection method may be performed withrespect to two or more tomographic images among the plurality oftomographic images, or at least one of the plurality of tomographicimages and at least one of the plurality of projection images Gi areprojected in the depth direction in which the tomographic planes of thesubject are arranged to generate the composite two-dimensional image.

In the fourth embodiment, the feature point detecting unit 33 firstdetects a two-dimensional feature point from the compositetwo-dimensional image. The detection of the two-dimensional featurepoint may be performed in the same manner as in the above embodiments.Then, the feature point detecting unit 33 detects the feature pointcorresponding to the two-dimensional feature point from the plurality oftomographic images Dj with reference to the depth map created inadvance.

The depth map is a map in which each position on the compositetwo-dimensional image is associated with the depth informationindicating the position of the tomographic plane corresponding to eachposition. The depth map is created by using the method disclosed inWO2014/203531A in advance. In the method disclosed in WO2014/203531A,first, the composite two-dimensional image is divided into a pluralityof local regions, and the correlation between the each region obtainedby division and the plurality of tomographic images Dj. For example, asshown in FIG. 22, the composite two-dimensional image C0 is divided into6×8 local regions, and the correlation between the plurality oftomographic images Dj and each of the divided regions is obtained. Then,the depth map is created by associating the depth of the tomographicimage Dj including the region with the largest correlation from thereference position of the tomographic plane with the position of eachregion. The reference position need only be, for example, the contactsurface of the breast M with the compression plate 17. Here, theposition of the tomographic plane Tj in a case of generating thetomographic image Dj is known. Therefore, by referring to the depth map,the position of the tomographic plane corresponding to each local regionin the composite two-dimensional image C0 can be specified.

In the fourth embodiment, the feature point detecting unit 33 identifiesthe tomographic plane of the detected two-dimensional feature point withreference to the depth map. Then, the feature point corresponding to thetwo-dimensional feature point is detected on the specified tomographicplane.

Here, the plurality of tomographic images Dj have a large amount ofinformation, so the amount of calculation for detecting the featurepoints is large. In the fourth embodiment, the two-dimensional featurepoint is detected from the composite two-dimensional image C0, and thefeature point corresponding to the two-dimensional feature point isdetected from the plurality of tomographic images Dj with reference tothe depth map. Therefore, in a case where the depth map is created inadvance, the amount of calculation can be reduced and the feature pointcan be detected quickly.

In the fourth embodiment, the display controller 36 may display thecomposite two-dimensional image on the display unit 3 together with thecorrected tomographic image.

Hereinafter, the fifth embodiment will be described. The configurationof the tomographic image generating apparatus according to the fifthembodiment is the same as the configuration of the tomographic imagegenerating apparatus according to the first embodiment shown in FIG. 3,only the processing to be performed is different, and thus the detaileddescription of the apparatus is omitted. The fifth embodiment isdifferent from the first embodiment in that the corrected tomographicimage Dhj is used as a new tomographic image, and feature pointdetection, tomographic plane projection image acquisition, positionalshift amount derivation, and new corrected tomographic image generationare repeated.

FIG. 23 is a flowchart showing a process performed in the fifthembodiment. In FIG. 23, the processing from step ST11 to step ST15 arethe same as the processing from step ST1 to step ST5 shown in FIG. 16,so detailed description thereof will be omitted here. In a case wherethe positional shift amount is derived in step ST15, the positionalshift amount derivation unit 35 determines whether the positional shiftamount converges (step ST16). The determination of whether thepositional shift amount converges may be performed by determiningwhether the positional shift amount derived for each tomographic planeprojection image GTi is equal to or smaller than a predeterminedthreshold Th1. The threshold Th1 may be set to a value at which it canbe said that there is no influence of body movement on the tomographicimage without correcting the positional shift amount any more. Thedetermination of whether the positional shift amount converges may beperformed by determining whether the average value of the positionalshift amounts derived for the plurality of tomographic plane projectionimage GTi is equal to or smaller than a predetermined threshold Th1. Ina case of positive in step ST16, it is unnecessary to correct thepositional shift amount, and thus the display controller 36 displays thetomographic image (step ST17), and the processing is terminated.

In a case of negative in step ST16, the reconstruction unit 32reconstructs the plurality of projection images Gi while correcting thepositional shift amount, and thereby generates a corrected tomographicimage Dhj as a new tomographic image (step ST18). Returning to theprocessing of step ST13, the feature point detecting unit 33 detects thefeature point from the plurality of new tomographic images, in stepST14, the projection unit 34 acquires a plurality of new tomographicplane projection images, and the positional shift amount derivation unit35 derives a new positional shift amount between the plurality of newtomographic plane projection images in step ST15, and determines whetherthe positional shift amount is equal to or smaller than thepredetermined threshold Th1 in step ST16. The processing of step ST18and step ST13 to step ST15 is repeated until it is determined to bepositive in step ST16. Also, in a case where the corrected tomographicimage is generated as a new tomographic image, the tomographic image tobe displayed in step ST17 is a new tomographic image.

As described above, in the fifth embodiment, the derivation of the newpositional shift amount based on the new tomographic image is repeateduntil the positional shift amount converges. Therefore, the positionalshift due to the body movement can be removed more appropriately andefficiently, and it possible to acquire a high-quality tomographic imagein which the body movement is accurately corrected.

As described above, in the second embodiment to the fourth embodiment,the derivation of the new positional shift based on the new tomographicimage may be repeated until the positional shift amount converges as inthe fifth embodiment.

Further, in the above embodiments, the positional shift amount derivedby the positional shift amount derivation unit 35 is compared with apredetermined threshold, and only in a case where the positional shiftamount exceeds the threshold value, the tomographic image may bereconstructed while correcting the positional shift amount. Thethreshold may be set to a value at which it can be said that there is noinfluence of body movement on the tomographic image without correctingthe positional shift amount. In this case, as shown in FIG. 24, awarning display 45 for notifying that the body movement exceeds thethreshold may be displayed on the display unit 3. The operator caninstruct whether to perform body movement correction by selecting YES orNO on the warning display 45.

In the above embodiments, in order to easily derive the positional shiftamount and the temporary positional shift amount, the regions ofinterest are set in the tomographic image Dj and the tomographic planeprojection image GTi, and the movement direction and the movement amountof the region of interest is derived as the shift vector, that is, thepositional shift amount and the temporary positional shift amount, butthe present invention is not limited thereto. The positional shiftamount may be derived without setting the region of interest.

Hereinafter, the sixth embodiment of the present disclosure will bedescribed. FIG. 25 is a diagram showing a schematic configuration of thetomographic image generating apparatus realized by installing atomographic image generating program in the computer 2 in the sixthembodiment. In FIG. 25, the same reference numbers as those in FIG. 3are assigned to the same configurations as those in FIG. 3, and detaileddescription thereof will be omitted here. The sixth embodiment isdifferent from the first embodiment in that the tomographic imagegenerating apparatus according to the sixth embodiment further comprisesa focal plane discrimination unit 38 that discriminates whether thecorresponding tomographic plane corresponding to the tomographic imagein which each of the plurality of feature points F is detected is afocal plane, and the positional shift amount derivation unit 35 derivesthe positional shift amount on the corresponding tomographic plane whichis discriminated to be the focal plane. The processing according to thesixth embodiment can be applied to the second to fifth embodiments, butonly the case where the processing is applied to the first embodimentwill be described here.

Here, in the tomographic image acquired by tomosynthesis imaging, thestructure glare occurs in the tomographic image other than thetomographic image in which the structure exists. This is called a rippleartifact. FIG. 26 is a diagram illustrating a ripple artifact. As shownin FIG. 26, assuming that a certain structure 48 is included in thetomographic image D3, the tomographic image corresponding to the upperand lower tomographic planes of the tomographic image D3 includes theripple artifact of the structure 48. The ripple artifact becomes widerand blurry as the distance from the tomographic plane including thestructure 48 increases. The range in which the ripple artifact spreadscorresponds to the range in which the X-ray source 16 moves.

Here, in a case where the feature point F detected by the feature pointdetecting unit 33 from the tomographic image Dj of the correspondingtomographic plane is the ripple artifact, the feature point F is blurredand spreads over a wide area. Therefore, in a case where such a featurepoint F is used, the positional shift amount cannot be derivedaccurately.

Therefore, in the sixth embodiment, the focal plane discrimination unit38 discriminates whether the corresponding tomographic plane the featurepoint F is detected is a focal plane, the projection unit 34 generatesthe tomographic plane projection image GTi on the correspondingtomographic plane which is discriminated to be the focal plane, and thepositional shift amount derivation unit 35 derives the positional shiftamount. Specifically, the positional shift amount is derived using thefeature point detected on the corresponding tomographic planediscriminated to be the focal plane. Hereinafter, the discrimination ofwhether the corresponding tomographic plane is the focal plane will bedescribed.

The focal plane discrimination unit 38 derives the correspondence pointcorresponding to the feature point in the plurality of tomographicimages for the feature point detected by the feature point detectingunit 33. FIG. 27 is a diagram illustrating the derivation ofcorrespondence points. As shown in FIG. 27, assuming that the featurepoint F3 is detected in a certain tomographic image Dk, the positionalshift amount derivation unit 35 derives the correspondence points R1,R2, R3, R4, . . . corresponding to the feature point F3 in the pluralityof tomographic images positioned in the thickness direction of thetomographic image Dk. In the following description, the reference codeof the correspondence point is R. The correspondence point R need onlybe derived by registration of the region of interest including thefeature point F3 with the tomographic image other than the tomographicimage Dk. Then, the focal plane discrimination unit 38 plots the pixelvalues of the feature point F3 and the correspondence point R in theorder in which the tomographic planes are arranged. FIG. 28 is a diagramshowing a result of plotting pixel values of the feature points and thecorrespondence points. As shown in FIG. 28, the pixel values of thefeature point and the correspondence point change so as to have aminimum value at the feature point due to the influence of the rippleartifact. Here, in a case where the feature point F3 is on the focalplane, the feature point F3 is not blurred, and the brightness is high,that is, the pixel value is small. On the other hand, in a case wherethe feature point F3 is not on the focal plane, the feature point F3 isthe ripple artifact, so that the pixel value is blurred and the pixelvalue is larger than the minimum value.

Therefore, the focal plane discrimination unit 38 discriminates that thecorresponding tomographic plane in which the feature point F3 isdetected is the focal plane in a case where the position of thetomographic plane in which the feature point F3 is detected is theposition P0 shown in FIG. 28 where the pixel value is the minimum in theresult of plotting the pixel values of the feature point F3 and thecorrespondence point. On the other hand, the focal plane discriminationunit 38 discriminates that the corresponding tomographic plane in whichthe feature point F3 is detected is not the focal plane in a case wherethe position of the tomographic plane where the feature point F3 isdetected is the position P1 or the like shown in FIG. 28 where the pixelvalue is not the minimum.

The projection unit 34 generates the tomographic plane projection imageGTi only on the corresponding tomographic plane discriminated to be thefocal plane, as in the above embodiments. The positional shift amountderivation unit 35 derives the positional shift amount of thetomographic plane projection image GTi on the corresponding tomographicplane discriminated to be the focal plane. That is, the positional shiftamount derivation unit 35 derives the positional shift amount of thetomographic plane projection image GTi by using the feature pointdetected on the corresponding tomographic plane discriminated to be thefocal plane.

Next, the processing performed in the sixth embodiment will bedescribed. FIG. 29 is a flowchart showing a process performed in thesixth embodiment. In FIG. 29, the processing from step ST21 to step ST23are the same as the processing from step ST1 to step ST3 shown in FIG.16, so detailed description thereof will be omitted here. In the sixthembodiment, it is assumed that the plurality of feature points aredetected. In a case where the feature point detecting unit 33 detectsthe plurality of feature points, the focal plane discrimination unit 38discriminates whether the corresponding tomographic plane correspondingto the tomographic image in which each of the plurality of featurepoints detected by the feature point detecting unit 33 is detected isthe focal plane (focal plane discrimination; step ST24). The projectionunit 34 generates the tomographic plane projection image GTi on thecorresponding tomographic plane discriminated to be the focal plane(step ST25), and the positional shift amount derivation unit 35 derivesthe positional shift amount by using the feature point detected in thecorresponding tomographic plane discriminated to be the focal plane(step ST26).

Further, the reconstruction unit 32 reconstructs the plurality ofprojection images Gi while correcting the positional shift amount, andthereby generates a corrected tomographic image Dhj (step ST27). Then,the display controller 36 displays the corrected tomographic image Dhjon the display unit 3 (step ST28), and the processing is terminated. Thegenerated corrected tomographic image Dhj is transmitted to the externalstorage device (not shown) and stored.

As described above, in the sixth embodiment, the positional shift amountis derived on the corresponding tomographic plane discriminated to bethe focal plane. Therefore, the positional shift amount can be derivedaccurately without being affected by the ripple artifact, and as aresult, the corrected tomographic image Dhj in which the positionalshift is accurately corrected can be generated.

In the sixth embodiment, discrimination is made as to whether thecorresponding tomographic plane is the focal plane by using the plotresults of the pixel values of the feature point and the correspondencepoint, but the discrimination of the focal plane is not limited thereto.Regarding the feature point and the ripple artifact, the difference incontrast of the feature point with the peripheral pixels is larger.Therefore, the contrasts of the feature point and the correspondencepoint with the peripheral pixels are derived, and in a case where thecontrast of the feature point is the maximum, the correspondingtomographic plane in which the feature point is detected may bediscriminated to be the focal plane. Further, the pixel value of theposition corresponding to the feature point in the projection image hasa small variation between the projection images in a case where thefeature point is on the focal plane, but in a case where the featurepoint is not on the focal plane, the projection image may represent thestructure other than the structure corresponding to the feature point,so the variation between the projection images is large. Therefore, thevariance value of the pixel value corresponding to the feature pointbetween the projection images Gi is derived, and in a case where thevariance value is equal to or smaller than a predetermined threshold,the corresponding tomographic plane in which the feature point isdetected may be discriminated to be the focal plane. Also, the focalplane discrimination unit 38 may include a discriminator that is machinelearned such that in a case where the pixel values of the feature pointand surrounding of the feature point are input, the discriminationresult is output as to whether the corresponding tomographic plane inwhich the feature point is detected is the focal plane. Thediscriminator may discriminate whether the corresponding tomographicplane in which the feature point is detected is the focal plane.

Hereinafter, the seventh embodiment of the present disclosure will bedescribed. FIG. 30 is a diagram showing a schematic configuration of thetomographic image generating apparatus realized by installing atomographic image generating program in the computer 2 in the seventhembodiment. In FIG. 30, the same reference numbers as those in FIG. 3are assigned to the same configurations as those in FIG. 3, and detaileddescription thereof will be omitted here. The seventh embodiment isdifferent from the first embodiment in that the tomographic imagegenerating apparatus further comprises a positional shift amountdetermination unit 39 that performs image quality evaluation for aregion of interest including the feature point in the correctedtomographic image Dhj, and determines whether the derived positionalshift amount is appropriate or inappropriate based on a result of theimage quality evaluation. The processing according to the seventhembodiment can be applied to the second to sixth embodiments, but onlythe case where the processing is applied to the first embodiment will bedescribed here.

The positional shift amount determination unit 39 sets, for the imagequality evaluation, the regions of interest Rh1 and Rh2 centered on thecoordinate positions of the plurality (here, two) of the feature pointsF4 and F5 included in the corrected tomographic image Dhj shown in FIG.31. Then, a high-frequency image is generated by extractinghigh-frequency components in each of the regions of interest Rh1 andRh2. The extraction of the high-frequency components need only beperformed by performing the filtering processing using the Laplacianfilter to generate a secondary differential image, but the presentinvention is not limited thereto. The positional shift amountdetermination unit 39 derives the magnitudes of the high-frequencycomponents of the regions of interest Rh1 and Rh2. The magnitude of thehigh-frequency components need only be derived by the sum of squares ofthe pixel value of the high-frequency image, but the present inventionis not limited thereto. The positional shift amount determination unit39 derives the sum of the magnitudes of the high-frequency components ofall of the regions of interest Rh1 and Rh2.

In a case where the positional shift correction is appropriatelyperformed by deriving the positional shift amount appropriately, theimage blurriness of the corrected tomographic image Dhj decreases, andthe high-frequency components increase. On the other hand, in a casewhere the positional shift correction is inappropriate due to theinappropriate derived positional shift amount, the image blurriness ofthe corrected tomographic image Dhj increases, and the high-frequencycomponents decrease. Therefore, in the seventh embodiment, thepositional shift amount determination unit 39 performs the image qualityevaluation based on the magnitude of the high-frequency components. Thatis, the positional shift amount determination unit 39 determines whetherthe sum of the magnitudes of the high-frequency components of all of theregions of interest Rh1 and Rh2, which are derived as above, is equal toor larger than the predetermined threshold Th2. In a case where the sumis equal to or larger than the threshold Th2, the positional shiftamount determination unit 39 determines that the positional shift amountis appropriate, and in a case where the sum is smaller than thethreshold Th2, the positional shift amount determination unit 39determines that the positional shift amount is inappropriate. In a casewhere the positional shift amount determination unit 39 determines thatthe positional shift amount is inappropriate, the display controller 36displays the tomographic image Dj before correction on the display unit3 instead of the corrected tomographic image Dhj. In this case, insteadof the corrected tomographic image Dhj, the tomographic image Dj beforecorrection is transmitted to the external storage device.

Next, the processing performed in the seventh embodiment will bedescribed. FIG. 32 is a flowchart showing a process performed in theseventh embodiment. In FIG. 32, the processing from step ST31 to stepST36 are the same as the processing from step ST1 to step ST6 shown inFIG. 16, so detailed description thereof will be omitted here. In a casewhere the reconstruction unit 32 generates the corrected tomographicimage Dhj, the positional shift amount determination unit 39 performsimage quality evaluation for a region of interest including the featurepoint in the corrected tomographic image Dhj, and determines whether thederived positional shift amount is appropriate or inappropriate based ona result of the image quality evaluation (step ST37).

In a case where the positional shift amount is appropriate, the displaycontroller 36 displays the corrected tomographic image Dhj on thedisplay unit 3 (step ST38), and the processing is terminated. Thegenerated corrected tomographic image Dhj is transmitted to the externalstorage device (not shown) and stored. On the other hand, in a casewhere the positional shift amount is inappropriate, the displaycontroller 36 displays the tomographic image Dj on the display unit 3(step ST39), and the processing is terminated. In this case, thetomographic image Dj is transmitted to the external storage device (notshown) and stored.

In a case where the positional shift amount is derived by the positionalshift amount derivation unit 35, an appropriate positional shift amountmay not be derived due to the influence of the structure other than thefeature point. In the seventh embodiment, the image quality evaluationis performed on the corrected tomographic image Dhj, and thedetermination is made as to whether the positional shift amount isappropriate or inappropriate based on the result of the image qualityevaluation. Therefore, it is possible to appropriately determine whetherthe derived positional shift amount is appropriate or inappropriate.Also, the tomographic image Dj before correction is displayed or storedin a case where the determination is made that the positional shiftamount is inappropriate, it is possible to reduce the possibility ofmaking an erroneous diagnosis due to the corrected tomographic image Dhjgenerated based on the inappropriate positional shift amount.

In the seventh embodiment, the image quality evaluation is performedbased on the magnitude of the high-frequency components of the region ofinterest set in the corrected tomographic image Dhj, but the presentinvention is not limited thereto. The positional shift amountdetermination unit 39 may perform the image quality evaluation for theregion of interest including the feature point in the tomographic imageDj, compare the result of the image quality evaluation for the correctedtomographic image Dhj with a result of the image quality evaluation forthe tomographic image Dj, and decide the tomographic image with a betterresult of the image quality evaluation as a final tomographic image.Here, the final tomographic image is the tomographic image that isdisplayed on the display unit 3, or transmitted and stored in theexternal device.

As described above, in the sixth embodiment to the seventh embodiment,the derivation of the new positional shift based on the new tomographicimage may be repeated until the positional shift amount converges as inthe fifth embodiment.

Further, also in the sixth embodiment and the seventh embodiment, thepositional shift amount derived by the positional shift amountderivation unit 35 is compared with a predetermined threshold, and onlyin a case where the positional shift amount exceeds the threshold value,the tomographic image may be reconstructed while correcting thepositional shift amount.

Hereinafter, the eighth embodiment of the present disclosure will bedescribed. FIG. 33 is a diagram showing a schematic configuration of thetomographic image generating apparatus realized by installing atomographic image generating program in the computer 2 in the eighthembodiment. In FIG. 33, the same reference numbers as those in FIG. 3are assigned to the same configurations as those in FIG. 3, and detaileddescription thereof will be omitted here. The eighth embodiment isdifferent from the first embodiment in that the tomographic imagegenerating apparatus further comprises an evaluation function derivationunit 50 that derives an evaluation function for performing image qualityevaluation for a region of interest including the feature point in thecorrected tomographic image Dhj, and the positional shift amountderivation unit 35 derives the positional shift amount for optimizingthe evaluation function. The processing according to the eighthembodiment can be applied to the second to sixth embodiments, but onlythe case where the processing is applied to the first embodiment will bedescribed here.

In the eighth embodiment, the evaluation function derivation unit 50generates the high-frequency image for the region of interestcorresponding to the feature point F, which is set with respect to thetomographic plane projection image GTi by the positional shift amountderivation unit 35. The generation of the high-frequency image need onlybe performed, as in the positional shift amount determination unit 39according to the seventh embodiment, by performing the filteringprocessing using the Laplacian filter to generate a secondarydifferential image. The pixel value of the derived high-frequency imagein the region of interest is referred to as qkl. k represents the k-thprojection image, and l represents the number of pixels in the region ofinterest.

Here, the transformation matrix for correcting the positional shiftamount is Wk, and the transformation parameter in the transformationmatrix is θk. The transformation parameter θk corresponds to thepositional shift amount. In this case, the image quality evaluationvalue of the region of interest corresponding to the feature point F inthe corrected tomographic image Dhj can be regarded as an added value ofthe magnitudes of the high-frequency image of the region of interestafter positional shift correction in each of the projection images Gi.By deriving the transformation parameter θk, that is, the positionalshift amount so that the added value is maximum, it is possible togenerate the corrected tomographic image Dhj in which the positionalshift amount is appropriately corrected.

Therefore, the evaluation function derivation unit 50 derives theevaluation function shown in Equation (3) below. The evaluation functionEc shown in Equation (3) is an evaluation function Ec to obtain thetransformation parameter θk for minimizing the value in parentheses onthe right side with a minus in order to maximize the above additionresult. The evaluation function shown in Equation (3) has a plurality oflocal solutions. Therefore, a constraint condition is applied to therange and the average value of the transformation parameter θk. Forexample, a constraint condition is applied such that the average of thetransformation parameters θk for all of the projection images is 0. Morespecifically, in a case where the transformation parameter θk is amovement vector representing parallel movement, a constraint conditionis applied in which the average value of the movement vectors for all ofthe projection images Gi is set to 0. Then, in the eighth embodiment,the positional shift amount derivation unit 35 derives thetransformation parameter θk to minimize the evaluation function Ec shownin Equation (3) below, that is, the positional shift amount.

$\begin{matrix}{{Ec} = {\arg \mspace{11mu} {\min\limits_{\theta}\left( {- {\sum\limits_{k}{\sum\limits_{l \in {ROI}}\left( {{{Wk}\left( {\theta \; k} \right)}{qkl}} \right)^{2}}}} \right)}}} & (3)\end{matrix}$

As described above, in the eighth embodiment, the tomographic imagegenerating apparatus further comprises an evaluation function derivationunit 50 that derives an evaluation function for performing image qualityevaluation for a region of interest including the feature point in thecorrected tomographic image Dhj, and the positional shift amountderivation unit 35 derives the positional shift amount for optimizingthe evaluation function. Therefore, it is possible to reduce thepossibility that an erroneous diagnosis is made by the correctedtomographic image Dhj generated based on the inappropriate positionalshift amount.

In the above embodiments, in order to easily derive the positional shiftamount and the temporary positional shift amount, the regions ofinterest are set in the tomographic image Dj and the tomographic planeprojection image GTi, and the movement direction and the movement amountof the region of interest is derived as the shift vector, that is, thepositional shift amount and the temporary positional shift amount, butthe present invention is not limited thereto. The positional shiftamount may be derived without setting the region of interest.

Further, in the above embodiments, the tomographic plane projectionimage GTi is acquired by the projection unit 34, and the positionalshift amount between the tomographic plane projection images GTi isderived by the positional shift amount derivation unit 35, but thepresent invention is limited to thereto. The positional shift amountbetween the projection images Gi may be derived without acquiring thetomographic plane projection image GTi. In this case, the projectionunit 34 is unnecessary in the above embodiments. The positional shiftamount derivation unit 35 need only derive the positional shift amountbased on the positional relationship of the projection images Gi on thecorresponding tomographic plane corresponding to the tomographic imagein which the feature point F is detected.

In the embodiments described above, the subject is the breast M, but thepresent invention is not limited thereto. It is needless to say that anypart such as the chest or the abdomen of the human body may be thesubject.

In the embodiments described above, for example, various processorsshown below can be used as the hardware structures of processing unitsthat execute various kinds of processing, such as the image acquisitionunit 31, the reconstruction unit 32, the feature point detecting unit33, the projection unit 34, the positional shift amount derivation unit35, the display controller 36, the combining unit 37, the focal planediscrimination unit 38, the positional shift amount determination unit39, and the evaluation function derivation unit 50. The variousprocessors include not only the above-described CPU, which is ageneral-purpose processor that executes software (program) and functionsas various processing units, but also a programmable logic device (PLD)that is a processor whose circuit configuration can be changed aftermanufacture, such as a field programmable gate array (FPGA), and adedicated electric circuit that is a processor having a circuitconfiguration that is designed for exclusive use in order to executespecific processing, such as an application specific integrated circuit(ASIC).

One processing unit may be configured by one of the various processors,or may be a combination of two or more processors of the same type ordifferent types (for example, a combination of a plurality of FPGAs or acombination of a CPU and an FPGA). Alternatively, a plurality ofprocessing units may be configured by one processor.

As an example of configuring a plurality of processing units by oneprocessor, first, as represented by a computer, such as a client and aserver, there is a form in which one processor is configured by acombination of one or more CPUs and software and this processorfunctions as a plurality of processing units. Second, as represented bya system on chip (SoC) or the like, there is a form of using a processorfor realizing the function of the entire system including a plurality ofprocessing units with one integrated circuit (IC) chip. Thus, variousprocessing units are configured by one or more of the above-describedvarious processors as a hardware structure.

More specifically, as the hardware structure of these variousprocessors, it is possible to use an electrical circuit (circuitry) inwhich circuit elements such as semiconductor elements are combined.

What is claimed is:
 1. A tomographic image generating apparatuscomprising at least one processor, wherein the processor is configuredto: acquire a plurality of projection images corresponding to aplurality of radiation source positions, the plurality of projectionimages being generated by causing an imaging apparatus to performtomosynthesis imaging in which a radiation source is moved relative to adetection surface of a detection unit in order to emit radiation to asubject at the plurality of radiation source positions according tomovement of the radiation source; reconstruct all or a part of theplurality of projection images to generate a tomographic image on eachof a plurality of tomographic planes of the subject; detect at least onefeature point from a plurality of the tomographic images; and derive apositional shift amount between the plurality of projection images basedon body movement of the subject with the feature point as a reference ona corresponding tomographic plane corresponding to the tomographic imagein which the feature point is detected, wherein the reconstruction unitreconstructs the plurality of projection images by correcting thepositional shift amount to generate a corrected tomographic image on atleast one tomographic plane of the subject.
 2. The tomographic imagegenerating apparatus according to claim 1, wherein the processor isconfigured to project the plurality of projection images on thecorresponding tomographic plane based on a positional relationshipbetween the radiation source position and the detection unit in a caseof imaging the plurality of projection images to acquire a tomographicplane projection image corresponding to each of the plurality ofprojection images, derive, as the positional shift amount between theplurality of projection images, a positional shift amount between aplurality of the tomographic plane projection images based on the bodymovement of the subject with the feature point as a reference on thecorresponding tomographic plane.
 3. The tomographic image generatingapparatus according to claim 2, wherein the processor is configured toset a local region corresponding to the feature point in the pluralityof tomographic plane projection images, and derives the positional shiftamount based on the local region.
 4. The tomographic image generatingapparatus according to claim 2, wherein the processor is configured toset a plurality of first local regions including the feature point inthe plurality of tomographic plane projection images, sets a secondlocal region including the feature point in the tomographic image inwhich the feature point is detected, derives a positional shift amountof each of the plurality of first local regions with respect to thesecond local region as a temporary positional shift amount, and derivesthe positional shift amount based on a plurality of the temporarypositional shift amounts.
 5. The tomographic image generating apparatusaccording to claim 4, wherein the processor is configured to derive thetemporary positional shift amount based on a peripheral region of thefeature point in the second local region.
 6. The tomographic imagegenerating apparatus according to claim 4, wherein the processor isconfigured to reconstruct the plurality of projection images excluding atarget projection image which corresponds to a target tomographic planeprojection image of which the positional shift amount is to be derived,and generates the plurality of tomographic images as target tomographicimages, and derive the positional shift amount of the target tomographicplane projection image by using the target tomographic images.
 7. Thetomographic image generating apparatus according to claim 1, wherein theprocessor is configured to detect a plurality of the feature points fromthe plurality of tomographic images, discriminate whether thecorresponding tomographic plane corresponding to the tomographic imagein which each of the plurality of feature points is detected is a focalplane, and derive the positional shift amount on the correspondingtomographic plane which is discriminated to be the focal plane.
 8. Thetomographic image generating apparatus according to claim 1, wherein theprocessor is configured to combine two or more tomographic images amongthe plurality of tomographic images to generate a compositetwo-dimensional image, and detect a two-dimensional feature point in thecomposite two-dimensional image, and detects the feature pointcorresponding to the two-dimensional feature point from the plurality oftomographic images.
 9. The tomographic image generating apparatusaccording to claim 1, wherein the processor is configured to reconstructall or a part of the plurality of projection images while correcting thepositional shift amount to generate a plurality of the correctedtomographic images on the plurality of tomographic planes of the subjectas a plurality of new tomographic images, detect the feature point fromthe plurality of new tomographic images, derive a new positional shiftamount between the plurality of new projection images, and reconstructthe plurality of projection images while correcting the new positionalshift amount to generate a new corrected tomographic image on at leastone tomographic plane of the subject.
 10. The tomographic imagegenerating apparatus according to claim 9, wherein the processor isconfigured to repeat generating of the new tomographic image, detectingof the feature point from the new tomographic image, and deriving of thenew positional shift amount until the new positional shift amountconverges.
 11. The tomographic image generating apparatus according toclaim 1, wherein the processor is configured to perform image qualityevaluation for a region of interest including the feature point in thecorrected tomographic image, and determines whether the derivedpositional shift amount is appropriate or inappropriate based on aresult of the image quality evaluation.
 12. The tomographic imagegenerating apparatus according to claim 11, wherein the processor isconfigured to perform the image quality evaluation for the region ofinterest including the feature point in the tomographic image, comparesthe result of the image quality evaluation for the corrected tomographicimage with a result of the image quality evaluation for the tomographicimage, and decides the tomographic image with a better result of theimage quality evaluation as a final tomographic image.
 13. Thetomographic image generating apparatus according to claim 1, wherein theprocessor is configured to derive an evaluation function for performingimage quality evaluation for a region of interest including the featurepoint in the corrected tomographic image, and derive the positionalshift amount for optimizing the evaluation function.
 14. The tomographicimage generating apparatus according to claim 1, wherein the subject isa breast.
 15. The tomographic image generating apparatus according toclaim 14, wherein the processor is configured to change a search rangein a case of deriving the positional shift amount depending on at leastone of a density of a mammary gland, a size of the breast, an imagingtime of the tomosynthesis imaging, a compression pressure of the breastin a case of the tomosynthesis imaging, or an imaging direction of thebreast.
 16. A tomographic image generating method comprising: acquiringa plurality of projection images corresponding to a plurality ofradiation source positions, the plurality of projection images beinggenerated by causing an imaging apparatus to perform tomosynthesisimaging in which a radiation source is moved relative to a detectionsurface of a detection unit in order to emit radiation to a subject atthe plurality of radiation source positions according to movement of theradiation source; reconstructing all or a part of the plurality ofprojection images to generate a tomographic image on each of a pluralityof tomographic planes of the subject; detecting at least one featurepoint from a plurality of the tomographic images; deriving a positionalshift amount between the plurality of projection images based on bodymovement of the subject with the feature point as a reference on acorresponding tomographic plane corresponding to the tomographic imagein which the feature point is detected; and reconstructing the pluralityof projection images by correcting the positional shift amount togenerate a corrected tomographic image on at least one tomographic planeof the subject.
 17. A non-transitory computer-readable storage mediumthat stores a tomographic image generating program causing a computer toexecute: a step of acquiring a plurality of projection imagescorresponding to a plurality of radiation source positions, theplurality of projection images being generated by causing an imagingapparatus to perform tomosynthesis imaging in which a radiation sourceis moved relative to a detection surface of a detection unit in order toemit radiation to a subject at the plurality of radiation sourcepositions according to movement of the radiation source; a step ofreconstructing all or a part of the plurality of projection images togenerate a tomographic image on each of a plurality of tomographicplanes of the subject; a step of detecting at least one feature pointfrom a plurality of the tomographic images; a step of deriving apositional shift amount between the plurality of projection images basedon body movement of the subject with the feature point as a reference ona corresponding tomographic plane corresponding to the tomographic imagein which the feature point is detected; and a step of reconstructing theplurality of projection images by correcting the positional shift amountto generate a corrected tomographic image on at least one tomographicplane of the subject.